What is the Preimplantation Genetic Diagnosis?

The preimplantation genetic diagnosis or PGD is a technique that allows experts in assisted reproduction to detect possible future genetic diseases and congenital malformations of the baby. We are, therefore, faced with an invaluable prevention method to avoid chromosomal abnormalities such as down syndrome, one of the most worrying situations for mothers, especially if they are over 40 years old.

The term PGD has begun to be known thanks to the rise of news related to infertility. It is complementary to one of the treatments, In Vitro Fertilization. In IVF, the ovules are fertilized outside the uterus, hence the name, in Vitro. From this process several embryos are born, which will later be implanted in the maternal womb in order to achieve the expected pregnancy.

This way, the embryos selected to transfer to the uterus, will be healthy, free of those genes that allow perpetuate diseases such as cancer. This selection takes place on the third day of the development of the embryo, when it has approximately eight identical cells or blastomeres, each of which contains all the necessary genetic information.

Not all In Vitro Fertilization cycles include preimplantation genetic diagnosis. It has been shown that those who do not guarantee not only a better health of the future baby, but also fewer spontaneous abortion rates and more success, therefore, in the final outcome of the treatment. A key of this guarantee that allows the sex of the embryo, a circumstance that helps prevent transmission of certain congenital diseases affecting a specific genre, such as hemophilia, cystic fibrosis, muscular dystrophy or Huntington disease.

Many couples opt for preimplantation genetic diagnosis, once they decide to undergo In Vitro Fertilization, for reasons of tranquility, to spend a more relaxed pregnancy in terms of possible spontaneous abortions and the health of the expected child. It is a personal decision. However, recommend doing it in the following cases:

When the male has alterations of the meiosis of the sperm (Sperm cells reduce their chromosome production by half).

For couples at risk of transmitting chromosomal alterations.

When there is a clinical history of recurrent abortions.

For couples with a clinical history of implantation failure after several attempts of In Vitro Fertilization.

Of course, also for women who have passed the limit of the so-called reproductive age, of 35 years, age at which the genetic material begins to grow old and there is a risk of chromosomal alterations in the baby.

What are the risks of PGD?

Limitations of genetic diagnosis: All diagnostic tests are aimed at establishing the normality or abnormality of one or several characteristics of the subjects studied, which are embryos in the case of PGD. Therefore, it is important to know that the normality of the results obtained with conventional PGD techniques does not exclude the existence of genetic anomalies or congenital ones not studied, not detectable or not identified by the applied procedures. For this reason, it is common that, if a pregnancy is achieved, it is recommended to supplement the information derived from PGD with prenatal diagnosis techniques such as amniocentesis or chorionic biopsy.

Non-conclusive diagnosis: Sometimes, although there are specific procedures for detecting genetic defects that are applicable and reliable for their pre-implantation use, the results obtained may not be conclusive, that is, they may not sufficiently inform about the normality or abnormality of one or several embryos with respect to the alteration investigated.

Insufficient number of embryos: As stated, the goal of PGD is to identify embryos not affected by an alteration to transfer them to the uterus and thus improve the probability of healthy offspring. If the number of available embryos is low, and in addition the risk of affectation due to the genetic disorder is high, it may be difficult to have an adequate number of transferable embryos. This circumstance is frequent in women with advanced age.

Embryonic deterioration: The extraction of the embryonic cells that have to be analyzed can negatively influence the development of some embryos. On the other hand, embryos must be kept in culture until obtaining diagnostic results that allow their selection, which may determine that certain embryos do not evolve adequately. Both circumstances can reduce the final number of embryos available for transfer. In extreme situations with a low initial number of embryos, this deterioration could affect all embryos or genetically normal embryos, making the transfer impossible.

What other diseases could PGD prevent?

In principle, we could be talking about 80 percent of rare diseases, which are those that have an identified genetic origin. Overall, there are more than 6,000 monogenic pathologies, each of them rare, but together they constitute a problem of the first magnitude due to the serious disorders they cause in patients and the physical and emotional burden they pose for families.

In addition, the latest advances in genetics allow detecting alterations in asymptomatic people, but who, for some cases occurred in their family, are suspected of being carriers of a disease. In this way, they have the option to make a selection of embryos before pregnancy and avoid passing that risk on to their children. This, which is already seen in patients at risk of having inherited Huntington’s disease, can be extended to many other degenerative pathologies, such as Alzheimer’s.